Technology Taking Shape – Radio Links

Control SystemFinalising ThunderStruck’s Radio Links

Aside from the airframe and servos, one of the hardest planning jobs is designing and building the various radio links.

It is pretty simple. Radio links are essential and not just nice. They will be mission critical to the success of the project, but we will have backups to complete the flight without crashing, etc. The links must be solid and with no breakup and must operate over long distances.

It is very important to realise the differences with the ground based systems and the aircraft systems. With the ground based systems we can have high power, large antennas, antenna tracking, mains/generator power and much more. on the aircraft we have both power and space issues. We also have temperature issues and the equipment must be tested in chambers that have had the air pumped out – I don’t like to use the term “vacuum”, but it is descriptive for most people.

How many links will we need?

At the moment we will need 4 radio links – 2 for the balloon and 2 for the aircraft.

  • The balloon telemetry system
  • The balloon camera system
  • The aircraft telemetry system
  • The aircraft camera system

We want to keep the video links separate from the telemetry as delays in the telemetry information can cause major issues. If you have ever had a large file download interrupt a Skype call?  you will know exactly what I mean. Imagine flying a supersonic aircraft and having dropouts on the links to the flight system! We can’t have that so we separate the systems. We also need to separate the balloon and aircraft systems as we will need to maintain video from the balloon well after the aircraft has separated from the balloon. We will also need to command the balloon to terminate its flight after separation. The most critical link of the 4 is the aircraft telemetry system and we have chosen a 900MHz 1 watt system. It is pretty amazing and handles 56Kb per second both ways at a distance of 80Km with diversity. Diversity is super important. I have posted the specifications on and earlier post, but I will repost them below. It can link directly to our control system and also to a navigation system such as the Pixhawk that we have chosen. The simple set up can be seen in the following diagram. More on this and the other links in a later post.

Control System

Note that in the above radio link system, the yagi antennas may have auto-tracking and will probably be vertical and horizontal diversity. We are toying with the idea of circular polarisation. More on patch antennas later.

From the RFDesign Website:

RFDesign is an electronics design and manufacturing company specialising in Embedded systems, Radios, Antennas and high frequency electronics. We are located in Brisbane, Australia with our office located in Acacia Ridge, QLD. 

Features:

  • Long range >40km depending on antennas and GCS setup
  • 2 x RP-SMA RF connectors, diversity switched.
  • 1 Watt (+30dBm) transmit power.
  • Transmit low pass filter.
  • > 20dB Low noise amplifier.
  • RX SAW filter.
  • Passive front end band pass filter.
  • Open source firmware SiK (V1.x) / tools, field upgradeable, easy to configure.
  • Multipoint software capability with MP SiK (V2.x)
  • Small, light weight.
  • Compatible with 3DR / Hope-RF radio modules.
  • License free use in Australia, Canada, USA, NZ

 Interfaces:

  • RF : 2 x RP-SMA connectors
  • Serial: Logic level TTL (+3.3v nominal, +5v tolerant)
  • Power: +5v, ~800mA max peak (at maximum transmit power)
  • GPIO: 6 General purpose IO (Digital, ADC, PWM capable).

Specifications:

  • Frequency Range:  902 – 928 MHz (USA) / 915 – 928 MHz (Australia)
  • Output Power: 1W (+30dBm), controllable in 1dB steps ( +/- 1dB @=20dBm typical )
  • Air Data transfer rates: 4, 8, 16, 19, 24, 32, 48, 64, 96, 128, 192 and 250 kbit/sec ( User selectable, 64k default )
  • UART data transfer rates: 2400, 4800, 9600, 19200, 38400, 57600, 115200 baud  ( User selectable, 57600 default )
  • Output Power: 1W (+30dBm)
  • Receive Sensitivity: >121 dBm at low data rates, high data rates (TBA)
  • Size: 30 mm (wide) x 57 mm (long) x 12.8 mm (thick) – Including RF Shield, Heatsink and connector extremeties
  • Weight: 14.5g
  • Mounting: 3 x M2.5 screws, 3 x header pin solder points
  • Power Supply: +5 V nominal, (+3.5 V min, +5.5 V max), ~800 mA peak at maximum power
  • Temp. Range: -40 to +85 deg C

Software / GCS Support:

The software solution is an open source development called “SiK” originally by Mike Smith and improved upon by Andrew Tridgell and RFDesign. A boot loader and interface is available for further development and field upgrade of the modem firmware via the serial port.

Most parameters are configurable via AT commands, Eg. baud rate (air/uart), frequency band, power levels, etc., please see the 3DR wiki for commands below for now.

V2.x firmware has been updated to support multipoint networking on the RFD900.

V1.x (non multipoint) is suitable for point to point links – the sourcecode is located at:   https://github.com/RFDesign/SiK

The user manual / datasheet can be found here : RFD900 Datasheet

A software manual for SiK firmware is here : RFD900 Software manual

RFD900 configuration tool: http://rfdesign.com.au/downloads/

RFD900 binary firmware repository: http://rfdesign.com.au/firmware/

3DR/RFD900 compatible configuration tool : http://vps.oborne.me/3drradioconfig.zip

Wiki for the 3DR radios (RFD900 has same commands): http://code.google.com/p/ardupilot-mega/wiki/3DRadio

Integrated support for configuring the RFD900 radios is supported by APM Planner, with other GCS solutions in development.

The default settings are at 57600 baud, N, 8, 1, and 64k air data rate.

Software features include:

  • Frequency hopping spread spectrum (FHSS)
  • Transparent serial link
  • Point to Point, or Multipoint networking
  • Configuration by simple AT commands for local radio, RT commands for remote radio
  • User configurable serial data rates and air datarates
  • Error correction routines, Mavlink protocol framing (user selectable)
  • Mavlink radio status reporting (Local RSSI, Remote RSSI, Local Noise, Remote Noise)
  • Automatic antenna diversity switching on a packet basis in realtime
  • Automatic duty cycle throttling based on radio temperature to avoid overheating

website, http://rfdesign.com.au for more information.

3 comments on “Technology Taking Shape – Radio Links

  1. Pingback: Adding a Cutdown to HABs | Real Space Adventure!

  2. Regarding these radio modules,You may have a hard time achieving 56k speeds at 80km. I vaguely recollect reading a forum entry where someone had created a 53km link. (No idea what their setup was). What you may find you need to reduce the baud rate to ensure the receivers are sensitive enough to pick up signal. Even at 9600baud at 35km, I occasionally dropped packets. The special ECC algorithm they use to try and ensure data integrity doesn’t always help. I created my own version of the firmware that did pack re-transmissions if no ACK was received. i.e. I didn’t bother with the ECC.

    That being said it these modems are a GREAT product, nothing quite like it out there and very affordable!

    • Yes, that is the nature of modems and the way that they add more data. the finer gained extra bits drop out as the distance is increased. I do not intend to get 56Kb data at the extreme distances, but that quickly changed as the aircraft flies towards the ground. We will not be directly under it, but we will see the distance reduce to a more respectable distance within the first minute of descent. There will be enough data rate to maintain control. On the uplink to the aircraft, it will only be commands from the remote control. The downlink will be GPS information each second – a very slow rate is okay. We will test to ensure that we do not have an issue. As an indication of the link speed changing, Voyager is not getting about 1.0 to 1.1 kbs at the moment. It certainly started off with a much higher data rate. The New Horizon craft that will fly by Pluto soon will have about a 1.8kps data rate and it will take months of continuous coverage with NASA’s set of 3 x 70m dishes (as the world turns) to bring back all the data collected in the pass. Good comment my firend and I will check the story you mentioned. We will be working with the manufacturer. We will also be working hard on an good antenna solution to increase the data rate.

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